It has been recognized that it is desirable to have the capability of selectively disabling or inhibiting deployment of the passenger-side and/or driver-side frontal air bags under circumstances where deployment could result in more harm than good. Accordingly, some automotive original equipment manufacturers (OEMs) have produced vehicles with key-operated cut-off switches for this purpose. Additionally, after-market products are available for retrofitting older vehicles to provide a similar cut-off function.
An example of an OEM SIR system with a cut-off switch is depicted in FIG. 1, where the reference numeral 10 designates an airbag controller (also referred to herein as a sensing and diagnostic module, or SDM) mounted in a protected, central portion of a vehicle. Operating voltage from a vehicle storage battery 12 is supplied to the SDM 10 via a key-operated ignition switch 14, and is designated as the ignition voltage V.sub.IGN. The SDM 10 includes an acceleration sensor AS for sensing longitudinal acceleration of the vehicle, a microprocessor .mu.P for receiving the output signal of acceleration sensor AS, and a pair of driver circuits 16, 18 coupled to squibs or initiators 20, 22 to deploy the driver-side and passenger-side frontal air bags (not shown) on command of microprocessor .mu.P in the event of a severe crash. The microprocessor .mu.P is typically coupled to a communications bus B, illustrated in FIG. 1 as a two-wire bus, via a bus interface module, as are other electronic modules such as the illustrated Body Computer. The SDM 10 may receive certain information such as vehicle speed over bus B, and may provide certain information to other modules via bus B. Additionally, the microprocessor may be coupled to an instrument panel lamp 24 via lamp driver LD for warning the driver of a system failure.
In the illustrated embodiment, deployment of the air bag associated with squib 22 (typically the passenger-side frontal air bag) requires enablement of driver circuit 18 by a Logic Circuit in addition to a triggering signal from microprocessor .mu.P. A key-operated single-throw double-pole cut-off switch 26 located external to SDM 10 is coupled to the SDM Logic Circuit to enable driver control of the cut-off function. In the normal switch configuration depicted in FIG. 1, the switch 26 couples the ignition voltage V.sub.IGN to Logic Circuit through a resistor 28. Logic Circuit recognizes this as an enabling condition, and enables driver circuit 18 so that the passenger-side frontal air bag can be deployed by microprocessor .mu.P in the event of a severe crash. However, when the key 30 is used to change the state of switch 26, the ignition voltage path through resistor 28 is opened, and the switch 26 couples the ignition voltage V.sub.IGN to a different input of Logic Circuit through a lamp 32, which illuminates to warn the vehicle occupants that the passenger-side frontal air bag is disabled. The Logic Circuit recognizes this as a cut-off activation, and removes the enabling voltage from driver circuit 18, effectively inhibiting deployment of the passenger-side frontal air bag.
FIG. 2 shows a different approach used to retrofit older vehicles that do not have a Logic Circuit of the type depicted in FIG. 1. In this case, a single-throw, double-pole switch 40 is inserted between the driver circuit 18 and the squib 22. In the normal switch position depicted in FIG. 2, the driver circuit 18 is connected across squib 22, whereas in the activated position, the driver circuit 18 is isolated from squib 22, and is instead connected across a resistor 42.
The two approaches discussed above are both disadvantageous in terms of cost. The OEM approach of FIG. 1 requires a special SDM 10 with additional circuitry (Logic Circuit) and I/O pins compared to a standard SDM, which increases cost and part proliferation. The after-market approach of FIG. 2 has no impact on the cost of the SDM, but requires an expensive, high current switch since the switch 40 is inserted directly in the firing loop of the passenger-side frontal air bag. These disadvantages become especially apparent, particularly in the after-market approach, when the cut-off function is needed for multiple air bags.